1. Field of the Invention
The present invention relates to a densifier for the simultaneous conditioning and densification of two cryogenic liquids, and more particularly to a fully acoustic densifier for the simultaneous densification of two cryogenic propellants at different temperatures.
2. Description of Related Art
Aerospace vehicles and spacecraft such as the space shuttle burn hydrogen fuel in the presence of oxygen for propulsion. To achieve maximum energy density and minimum storage volume, the hydrogen and oxygen propellants are stored onboard the spacecraft as cryogenic liquids. To achieve even greater energy density and lower volume, it is desirable to densify the cryogenic liquid propellants by subcooling or supercooling them below their normal boiling point temperatures.
Liquid oxygen normally (at 1 ATM) boils at 90.15 K and liquid hydrogen at 20.25 K. At their boiling points, liquid oxygen and liquid hydrogen have densities of approximately 1141 kg/m3 and 70 kg/m3 respectively. However, both oxygen and hydrogen can be densified by supercooling below their boiling points. A densified, supercooled propellant can be stored in a smaller volume and at lower pressure than an equivalent amount (mass) of a saturated liquid propellant.
In the case of a spacecraft or other types of aerospace vehicles, densification of propellants is desirable for at least three reasons. First, increased propellant density translates into smaller propellant tanks which result in lower take-off weight and larger payload capacities. Second, densified propellants require lower operating pressures in propellant tanks, thus extending tank life in reusable systems, lowering recurring costs and reducing life-cycle costs. In addition, lower operating pressures for expendable launch vehicles result in lower pressurizing gas requirements. Third, increased propellant density lowers turbo-machinery rotational speeds which increases reliability and safety, and reduces life-cycle costs for reusable systems.
A fourth potential benefit of supercooled, densified propellants is that the increased cooling capacity of the propellants themselves can provide a potentially vital heat sink for leading edge and shock wave regions of an aerospace vehicle resulting from aerodynamic heating, and for rocket or rocket-based combined cycle (RBCC) engine combustion chambers and nozzles.
Current apparatus and techniques for densifying cryogenic propellants suffer from a number of drawbacks, principal among which is that most require moving parts in a cryogenic refrigeration system. U.S. Pat. No. 5,644,920 describes a method of densifying liquid propellants via circulation through a low temperature cryogenic liquid bath which is maintained under vacuum by a rotary cold gas compressor. According to this method a mechanical machine having moving parts (the compressor) must operate adjacent to or in contact with cryogenic materials likely to cause machine failure. This system was tested and reported by NASA (Tomsik, T. M., “Performance Tests of a Liquid Hydrogen Propellant Densification Ground Support System for the X33/RLV”, AIAA-97-2976, July 1997) in a pilot-scale unit designed to densify liquid hydrogen (LH2) from 20 K to a supercooled temperature of about 16.1 K at a rate of 0.9 kg/sec for 60 seconds at steady-state. The test program was cancelled primarily due to failure of the compressor.
A second cold gas compressor apparatus as described above is currently being tested at the NASA Glenn Research Center in Cleveland, Ohio to densify liquid oxygen to support NASA's X-33 launch vehicle (the X-33 oxygen densifier). The X-33 oxygen densifier is designed to densify 13.6 kg/sec of liquid oxygen down to a supercooled temperature of about 67 K at steady state. Testing of the X-33 oxygen densifier has shown the cold gas compression units to be highly unstable, un-repeatable, and unreliable during operation for long periods of time; i.e. the time required to load a launch vehicle. In fact, one of the compressor stages of the X-33 oxygen densifier has failed causing destructive damage to the impeller and impeller housing.
Warm gas compressor systems have also been devised. These systems are similar to the cold gas compressor systems described above, except that warm gas compressors or vacuum pumps are used to create the evaporative cooling effect directly inside the storage tank of the cryogenic propellant. A heat exchanger is used to warm the evacuated vapor prior to entering the vacuum pumps because the pumps cannot handle cold vapors. This technique has been used effectively since the 1960's to make slush nitrogen and hydrogen, however it still requires moving parts and the input of mechanical energy at high cost.
Other known methods of cryogenic liquid propellant densification are described briefly below:
U.S. Pat. No. 6,164,078 teaches that fluid ejectors can be used to create sub-atmospheric pressures in a cryogenic fluid inside a heat exchanger reservoir. The ejector which has no moving parts performs the same function as the cold gas compressors discussed previously. U.S. Pat. No. 6,116,030 teaches the use of a specific ejector that uses steam as the primary motive force. The steam is generated as the combustion product of hydrogen and oxygen. Additional steam is generated by the addition of liquid water to the product steam. U.S. Pat. No. 6,151,900 teaches the use of a second cryogenic fluid to cool a first cryogenic fluid having a higher boiling point. The second cryogenic fluid is injected into the first cryogenic fluid causing the second cryogenic fluid to be vaporized and released through a vent. U.S. Pat. No. 6,131,395 teaches the use of boil-off vapors from a colder second fluid to cool a first cryogenic fluid through indirect heat exchange inside a container. The example given is using the boil-off vapors from gaseous hydrogen to densify liquid oxygen by flowing both fluids through a common heat exchanger. Safety is a concern with this system because a single-point failure between the tube walls of the heat exchangers would allow mixing of the hydrogen and oxygen streams. Turbo-Brayton Cycle Helium Refrigeration Systems are known to work in the temperature and heat-capacity range required for propellant densification systems. However, they too require rotating machinery operating at cryogenic temperatures. Likewise, Stirling cycle refrigerators, which have been used for a long time in cryogenic processes, also have at least two moving parts; a compressor and a displacer. The displacer is located at the cold end of the refrigerator, and is subject to cryogenic temperatures. Indeed, history has shown that cryogenic Stirling refrigeration systems require periodic maintenance to replace the moving parts, contributing to down-time and increased operation cost.
The major disadvantages of the above densification methods are poor reliability and high operational and maintenance costs associated with rotating machinery and moving parts. This is especially true when moving machinery is operated at cryogenic temperatures. The use of uncertain, or unreliable densification technologies for propellant densification is very risky for companies and governments that operate aerospace launch vehicles. This risk is unacceptable, especially when a launch vehicle must launch within a specified time window which may be as narrow as 10 minutes. Such narrow launch windows make intolerable any substantial risk of delay due to propellant densification system failure.
A key disadvantage of evaporative cooling techniques is the generation of sub-atmospheric pressures inside hydrogen storage tanks. This can lead to a potentially catastrophic situation in which air (oxygen) from the atmosphere is drawn into the hydrogen system through a leaky seal or a vent.
There is a need in the art for a system for densifying cryogenic propellants, such as hydrogen and oxygen, that has no moving parts. Preferably, such a system will be at least as efficient as existing densification systems but with no mechanical energy input, and preferably will be capable of simultaneously densifying two cryogenic propellants at two different temperatures.